1. Field of the Invention
The present invention relates generally to surge protectors of the type commonly used on telecommunications lines to divert voltage surges to ground, and more particularly, to a failsafe surge protector having a reduced part count.
2. Description of the Related Art
Surge protectors are well known for protecting personnel and telecommunications equipment by diverting voltage surges, also known as overvoltages, on a telecommunications line to ground. Such surge protectors utilize various types of protection elements to divert unacceptable levels of voltage to ground, including an air gap, a gas tube, and a metal oxide varistor (MOV) or other solid state device (e.g., thyristor or bi-directional voltage switch). A surge protector utilizing a single protection element is sometimes reliable and provides sufficient protection against the level of voltage surges encountered. Oftentimes, however, a surge protector utilizing more than one type of protection element is required to provide redundancy or to improve the performance of the surge protector. For example, an MOV may be used in conjunction with a gas tube as a back-up protection device to provide continued protection to personnel and equipment in the event that the gas tube fails (e.g., the gas tube vents). When used as a back-up protection device, the MOV has a clamping voltage at a preselected current (e.g., 1 mA) that is greater than the DC breakdown voltage of the gas tube. An MOV may also be used in conjunction with a gas tube as a hybrid protection device to reduce the reaction time of the surge protector or to reduce the impulse breakdown voltage of the gas tube without permitting the MOV to burn out. When used as a hybrid protection device, the MOV has a clamping voltage at a preselected current (e.g., 1 mA) that is less than the DC breakdown voltage of the gas tube.
It is also known to provide surge protectors with a failsafe mode of operation. A surge protector provided with a failsafe mode of operation continues to protect personnel and equipment in the event that the primary protection element overheats, or both the primary protection element and the secondary protection element overheat. In a particular failsafe surge protector, the protector assembly is provided with a fusible element having a predetermined melt temperature. If the temperature of the fusible element reaches the predetermined melt temperature, the fusible element melts and provides an electrical short-circuit path between the telecommunications line and ground. A commonly utilized fusible element is a solder pellet made of a fluxed metal alloy that has a predictable melt temperature and transitions rapidly between the solid state and the liquid state. The melt temperature of the fusible element is selected based on the temperature at which the protection element overheats (or is otherwise rendered inoperable), the thermal conductivity of the protection element, and the location of the fusible element in the surge protector relative to the protection element.
A known failsafe surge protector including a gas tube, an MOV, and a fusible solder pellet for protecting the tip and ring conductors of a telecommunications line is shown in FIGS. 1A and 1B. The surge protector, indicated generally at 110, includes a non-conductive housing 112 defining an internal cavity. Tip and ring line terminals 114 extend outwardly from the cavity through openings formed in the top surface of the housing 112. Each line terminal 114 is threaded to receive fasteners 113 and spacers 115 for securing and separating one or more tip and ring wire pairs that are electrically connected to the line terminals 114. The surge protector 110 further includes a pair of voluted springs 116, a pair of gas tube assemblies 118 (FIG. 1A), a common ground cap 119, and a common ground terminal 120. The voluted springs 116, the gas tube assemblies 118 and the ground cap 119 are disposed within the cavity defined by the housing 112 and are secured within the cavity by the legs 121 of the ground terminal 120, which engage the underside of the ground cap 119 through openings formed at the bottom of the housing 112. The underside of the ground cap 119 and the legs 121 of the ground terminal are typically encased with a potting compound (not shown) to seal the internal cavity along the bottom of the housing 112 against environmental contaminants, such as dirt, dust and moisture.
Each voluted spring 116 is electrically connected to one of the tip and ring line terminals 114. The gas tube assemblies 118 include identical sets of tip terminal protection elements and ring terminal protection elements that are electrically connected to a common ground support 122 (FIG. 1B). Each set of protection elements includes a gas tube 124, a pair of opposed end caps 126, a fusible solder pellet 127, an MOV 128, and a failsafe MOV spring 129 having a first end 131 and a second end 133. An elastic retaining band 130 holds the failsafe MOV spring 129 in position apart from the ground support 122, thereby preventing a short-circuit between the line terminal 114 and the ground terminal 120 through the failsafe MOV spring 129, the ground support 122, and the ground cap 119. The gas tube assemblies 118 are electrically connected between the voluted springs 116 and the ground cap 119, which in turn is electrically connected to the ground terminal 120. Thus, each conductor secured on a tip or ring line terminal 114 is electrically connected to the ground terminal 120 through one of the voluted springs 116, the corresponding gas tube assembly 118, the ground support 122, the ground cap 119, and the ground terminal 120.
The gas tube 124 is disposed between the ground support 122 and the failsafe MOV spring 129. The gas tube 124 has a first electrode 132 electrically connected to the ground support 122, and a second electrode 134 spaced from the first electrode that is electrically connected to the first end 131 of the failsafe MOV spring 129 adjacent the voluted spring 116. Accordingly, the surge protector 110 provides a first electrical ground path from the line terminal 114, through the voluted spring 116, through the first end 131 of the failsafe MOV spring 129, between the second electrode 134 and the first electrode 132 of the gas tube 124, through the ground support 122, through the ground cap 119, and out to the ground terminal 120.
The fusible solder pellet 127 and the MOV 128 are disposed between the opposed end caps 126. One of the end caps 126 is electrically connected to the ground support 122 while the other end cap 126 is electrically connected to the second end 133 of the failsafe MOV spring 129. Accordingly, the surge protector 110 provides a second electrical ground path from the line terminal 114, through the voluted spring 116, between the first end 131 and the second end 133 of the failsafe MOV spring 129, through the MOV 128 and the fusible solder pellet 127 between the opposed end caps 126, through the ground support 122, through the ground cap 119, and out to the ground terminal 120. The second electrical ground path is parallel to the first electrical ground path and diverts voltage surges to ground if the gas tube 124 fails (i.e., when the MOV 128 acts as a back-up protection device) or when the MOV 128 operates in conjunction with the gas tube 124 as a hybrid protection device.
In the event of a sustained voltage surge, the gas tube 124 and/or the MOV 128 will overheat, and thereby cause the fusible solder pellet 127 to melt. Once the fusible solder pellet 127 melts, the failsafe MOV spring 129 forces the outer edges of the opposed end caps 126 into contact with one another. Accordingly, the surge protector 110 provides an electrical short-circuit path parallel to both the first electrical ground path and the second electrical ground path from the line terminal 114, through the voluted spring 116, between the first end 131 and the second end 133 of the failsafe MOV spring 129, through the end caps 126, through the ground support 122, through the ground cap 119, and out to the ground terminal 120. Thus, the surge protector 110 provides three parallel electrical paths to divert voltage surges between one or more conductors on the tip and ring line terminals 114 and ground. The voltage surges may be carried to ground through the gas tube 124 along the first electrical ground path, through the MOV 128 and the fusible solder pellet 127 along the second electrical ground path, or may by-pass the gas tube 124, the MOV 128, and the fusible solder pellet 127 by traveling through the end caps 126 along the electrical short-circuit path. The MOV 128 may act as a back-up protection device, or may operate in conjunction with the gas tube 124 as a hybrid protection device to improve the performance of the surge protector 110.
The surge protector 110 is commonly referred to as a xe2x80x9cstation protection modulexe2x80x9d and is used within a protection, connection or termination enclosure, such as a network interface device (NID) or building entrance terminal (BET) on a telecommunications line to protect personnel and equipment from voltage surges caused, for example, by a lightening strike or power cross. Typically, a large number of telecommunications lines are interconnected within a NID or BET having significant volume constraints. As a result, it is desirable that the surge protector 110 for each telecommunication line occupies as little space as possible. A known shortcoming of such failsafe surge protectors is the large number of components that are required to adequately protect a telecommunications line. The number of components used to construct the failsafe surge protector 110 necessarily increases the space it occupies within a NID or BET. Furthermore, the number of components increases the complexity of the assembly process and the cost to manufacture the surge protector 110. Thus, it would be desirable to reduce the number of components (i.e., part count) used to construct the surge protector 110, thereby reducing the space (i.e., footprint) occupied by the surge protector within an interconnection enclosure, as well as the complexity of the assembly process and the cost to manufacture the surge protector. In addition, it is oftentimes necessary for each surge protector to provide redundancy or improved performance in addition to the failsafe mode of operation. What is needed is a failsafe surge protector having a reduced part count that may further include a secondary protection element to provide redundancy or to improve the performance of the surge protector.